Recently, owing to the trend of semiconductors toward higher integration and magnetic heads toward further miniaturization, the production units (such as an exposure meter, a processing machine, and a construction machine) for such semiconductors and magnetic heads, measuring devices, measuring prototypes, and reflecting mirrors have reached the point of requiring high dimensional accuracy and high rigidity. For these devices, the stability of dimensional accuracy has also come to gain insignificance. The prevention of such devices from incurring the deformation which is caused by the fluctuation of an ambient temperature or the emission of heat from the device itself has become an important task. The materials which produce very small thermal expansions and abound in rigidity and specific rigidity (Young's modulus/specific gravity) have come to find the use for component members in such devices.
The various devices as mentioned above are mostly aimed at handling the lights such as laser beam, ultraviolet light, and visible ray for the purpose of exposure or measurement. The members which are used in these devices more often than not abhor unnecessary reflection (including irregular reflection) or transmission of light. On many occasions, black materials which succumb only sparingly to reflection and transmission of light are found as necessary supplies.
The low thermal expansion materials which have heretofore known in the art include Invar alloy (Fe—Ni type) and super-Invar alloy (Fe—Ni—Co type) in the class of metals, such low thermal expansion glasses as ZERODUR™ glass (available from Schott ML GmbH in Germany), quartz glass (SiO2), and titanium dioxide-containing quartz glass (SiO2—TiO2) in the class of ceramics, aluminum titanate (TiO2—Al2O3), cordierite (MgO—Al2O3—SiO2) type sintered body and glass, lithium-alumino-silicate (Li2O—Al2O3—SiO2) type sintered body and glass.
The super-Invar alloy as a low thermal expansion metal indeed manifests such a low thermal expansion coefficient as 0.13×10−6/° C. at room temperature and yet has such a large specific gravity as 8.2 g/cm2 and such a not very high Young's modulus as 125 GPa. Thus, it has a very low specific rigidity approximately of 15 GPa cm3/g and, therefore, is deficient in mechanical stability. The term “specific rigidity” as used herein means a magnitude obtained by dividing a Young's modulus (E) by a specific gravity (ρ) (specific rigidity=E/ρ). The impartation of the black color to the surface of this alloy has no alternative but to rely on the method of using such a surface coat as a black Cr plating on the surface layer. The use of the surface coat, however, entails the problem of exerting adverse effects on the low thermal expansion property and the precision machining property.
The quartz glass, while enjoying such a low thermal expansion coefficient as 0.48×10−6/° C., suffers from such an insufficient specific rigidity as about 33 GPa cm3/g and a clear tone.
The ZERODUR™ glass has been finding the extensive utility in applications to such devices as measuring prototypes. It enjoys such a sufficiently low thermal expansion coefficient as 0.02×10−6/° C. at room temperature and yet suffers from a clear tone. It further encounters difficulty in forming products too complicate in shape and products too large in size to manufacture. Further, since it manifests specific rigidity and Young's modulus respectively approximating 35.6 GPa·cm3/g and 90 GPa, it does not fully fit the use aimed at by this invention.
As regards the aluminum titanate, it has been known to have produced a sintered body manifesting such a low thermal expansion coefficient as −0.068×10−6/° C. (“Glossary of Fine Ceramics Catalogs (1987)”, p. 140). This compound manifests such a high water absorption as 1.59% and thus offers only insufficient denseness for the use aimed at by this invention. No sintered bodies of this compound has been known to possess a black tone.
The lithium-alumino-silicate type sintered body and glass are deficient in mechanical stability because it manifests only such insufficiently high specific rigidity as not more than 33 GPa·cm3/g in spite of such a small thermal expansion coefficient as falls in the range of −5 to 1×10−6/° C. It has predominantly acquired a white tone and has not acquired a black tone so far.
JP-A-61-72, 679 discloses a low thermal expansion ceramic sintered body which has a chemical composition mainly comprising 0.3–5.5 mass % of Li2O, 4.1–16.4 mass % of MgO, 2.07–42.8 mass % of Al2O3, and 46.3–70.1 mass % of SiO2, a crystal phase containing not less than 30 mass % of cordierite and not less than 5 mass % of β-spodumene as main components, and manifests a thermal expansion coefficient of 2.0×10−6/° C. at a temperature in the range of 20° C.–800° C. This publication, however, has absolutely no mention of the tone of the sintered body and points out the fact that the sintered body produced by the method taught in the publication does not acquire a black tone (refer to Comparative Example 22 in Table 1 inserted in the working example which will be specifically described hereinbelow).
JP-A-10-53,460 reports a dense ceramic substance which comprises 1.5–6.5 mass % of Li2O, 1.0–10 mass % of MgO, 14–30 mass % of Al2O3, and 58–83 mass % of SiO2, and allows the coexistence of petalite, spodumene, and cordierite in a crystal phase and demonstrates that this substance excels in resistance to thermal shock. This publication, however, has absolutely no mention of the tone of the ceramic and points out the fact that the sintered body produced by the method taught in the publication does not acquire a black tone (refer to Comparative Example 23 in Table 1 inserted in the working example which will be specifically described hereinbelow).
“Ceramics”, Vol. 18 (1983) No. 5 discloses a Co—Cr—Fe type spinel, a Co—Mn—Fe type spinel, a Co—Mn—Cr—Fe type spinel, a Co—Ni—Cr—Fe type spinel, and a Co—Ni—Mn—Cr—Fe type spinel as black pigments for the use in coloring ceramics and also discloses a solid solution of Sb in SiO2 and a solid solution of Co and Ni in ZrSiO4 as gray pigments. These pigments, however, are intended to utilize the phenomenon of coloration in the graze on the surface of ceramics and not to impart a black color to a depth in the sintered body itself. Any attempt to use the graze on the surface of a low thermal expansion ceramic substance proves futile because the difference in thermal expansion between the ceramic substance and the graze tends to inflict a crack to the applied layer of the graze.
The silicon carbide sintered body has been commercially available as a black ceramic substance. JP-A-08-310,860 discloses a black zirconia ceramic sintered body, JP-A-04-50,161 a method for the production of a high rigidity black alumina sintered body, and JP-A-06-172,034 a black silicon nitride sintered body, respectively. Though these sintered bodies are black, their thermal expansion coefficients at room temperature are 2.3×10−6/° C. in the sintered body of silicon carbide, 7×10−6/° C. in that of zirconia, 5.3×10−6/° C. in that of alumina, and 1.3×10−6/° C. in that of silicon nitride. Thus, these sintered bodies are incapable of realizing a low thermal expansion coefficient aimed at this invention.
Incidentally, the term “room temperature” as used in this invention refers to the range of temperatures, 20° C.–25° C. The room temperature mentioned in the present specification invariably refers to this temperature range.
JP-B-57-29,436 discloses a technique which comprises adding to a cordierite sintered body an oxide of such a transition element as Zn, V, Nb, Cr, Mo, or W for the purpose of densifying the sintered body. The sintered body obtained by this technique, however, manifests such an insufficiently low thermal expansion coefficient as 0.96×10−6/° C., fails to acquire sufficient densification as evident from water absorption of 4.6%, and suffers from not sufficiently high rigidity. The publication has absolutely no mention of the tone.
Recently, JP-A-11-343,168 discloses a technique for the impartation of a black color to a ceramic substance containing not less than 80 mass % of cordierite by the incorporation of 0.1–2.0 mass % of carbon into the ceramic substance.
The invention disclosed in this publication is characterized by incorporating carbon and, therefore, is different from the present invention which does not need the incorporation of carbon.
The incorporation of carbon in a sintered body entails the problem of heightening the thermal expansion coefficient as indicated in the above publication, exerts such an adverse effect on mechanical properties as lowering the modulus of elasticity, renders the formation of products large in wall thickness and size difficult to attain by sintering, and inevitably imposes restrictions on the shapes of such products. Thus, the incorporation of carbon proves unfavorable.
The material which contains no carbon, assumes a black color, manifests low thermal expansion, and possesses rigidity and specific rigidity high enough to ensure effective use as building materials has not been known to date.
This invention is aimed at providing a black low thermal expansion ceramic sintered body which assumes a black tone and manifests very low thermal expansion and high rigidity and specific rigidity at room temperature and a method for the production thereof.